Minarelli 2T rings

[90GTVert]

Unless your cylinder was in rough shape or glazed I dunno that honing or scuffing would cure this problem.

[duosport]

The cylinder was/is in beautiful shape. I read that the honing was to put little scratches in the cylinder wall that would hold oil and act as an abrasve surface to shape the rings and seat them.

[duosport]

So I rode it again today to see if the rings would still seat. It seems to be riding slightly faster again today. On the flats it is going 27 mph. The comression tesat however stayed at 92 PSI. Is it possible that the rings can still seat? Or should I be looking to tear it apart now and investigate something else? I do not wish to put a BBK on this scooter. I really want to keep it stock. What should I do next?

[stepthrutuner]

Have you been doing any plug chops/reads? What is the power flow from take off to top speed like? How does it take the throttle? That scoot should be topping 40+ even with low compression I would think. Start with a new spark plug to do spark plug readings. Keep digging. Magic mark the inside of the pulleys to check belt travel out.

[duosport]

May 6, 2010 10:02:31 GMT -5 stepthrutuner said:

Have you been doing any plug chops/reads? What is the power flow from take off to top speed like? How does it take the throttle? That scoot should be topping 40+ even with low compression I would think. Start with a new spark plug to do spark plug readings. Keep digging. Magic mark the inside of the pulleys to check belt travel out.

I am really confused by these responses. I have found the problem in general. There is low compression. Checking the variator and such when I have an engine with 92PSI is like putting earings on a pig. If I said I was fixing the variator and I have low compression 92 PSI you would be saying "Duosport, what are you looking at the variator for?"

What I am hoping for is advice on fixing the low compression so that I then can do these other things.

[90GTVert]

I think a lot of the responses are because you yourself seemed uncertain about your compression tester. If you are convinced that it is correct, then your first step is to pull the cylinder back off. If it's that low, you aren't gonna fix it with any amount of driving. An engine that's never been started to seat the rings should make more than 92psi. If possible, take pics. At very least you need to start measuring piston ring end gap, bore diameter, piston diameter, inspecting the rings, inspecting the piston and ring lands, inspecting the cylinder walls, making sure the deck and cylinder head mating surfaces are true. There really isn't anything you can do from outside the engine to boost compression. Gotta get in there and see what's going on.

[duosport]

Thanks for that. I have since tested the compression on my old lawn mower with the gage. It tested out at 115 PSI. I feel pretty confident that the gage is not in error. I have a new piston and new rings so that the problem would have to be the cylinder or the head. I gather measurements compared to specs is my next step. Check head for flatness as well.

[stepthrutuner]

Here's the deal. If you are able to determine that your bore is worn too big for your piston you're going to have to get a new piston after the expense of having your cylinder professionally bored. The best tack, then, would be to just buy a new original size cylinder to go with your new piston. Probably you should buy it from the same supplier and with the same brand as your piston. Just buying the cylinder may prove to be almost as expensive as buying a whole new cylinder/piston set. This might be the best deal and just keep the new piston you already have as a spare - just in case.

[duosport]

OK I have again disassembled the top end. I will go no further than giving the visual description of what I see. I di not have access to a digital camera now. The cylinder looks OK. There is some slight darkening of the bore. Perhaps this is considered glazing but it does not look glazed to my eye. I see no scratches or anything. The piston is another story. This is my new replacement piston and not original minarelli. There are some carbon deposits in the center top of the piston. They run down to the edge on one side. It is right where the arrow is. Jost over the edge in that spot there are carbon deposits on the sides above the first ring and they go around about 1/3 of the way. There are also some deposits above the end gap of the top ring also. Below the top ring there are no deposits. At the very bottom of the sides of the piston there are some small small scratches. They run from the bottom to the mid point. The scratches run verticle and look as though someone took some 100 grit sandpaper and gave it a stoke or two with it. To note, I do not have these scratches in my original piston.

I know this is only the visual description. I can check for flatness of the head (which looks good by the way) and of the top of the cylinder. I do not have a micrometer nor a bore guage. Perhaps I can take these to a mechanic to have mesured.

[90GTVert]

The piston doesn't sound bad other than the scratches. The buildup sounds pretty normal to me. If the carbon was building up below the rings that would indicate that gases were passing the rings and that could cause lower compression, but it sounds like that's not an issue here.

If you've got a Harbor Freight or similar store nearby you can get a set of calipers pretty cheap...
www.harborfreight.com/6-inch-composite-digital-caliper-93293.html

[duosport]

Correct me if I am wrong but I would need to measure down into the bore aswell as the top right?

[90GTVert]

Yeah. Any measurement you can get is helpful. The fact that you aren't getting blowby according to your piston is making me doubt your compression tester again though.

[duosport]

Well I used that compression tester on my 7 year old push lawn mower. I got a 115 PSI reading on it. That seems pretty normal for an old (running) 4 HP Briggs and Straton doesn't it?

[duosport]

I have not yet checked the flatness of the head and top of cylinder. These could certainly be cause of loss of compression too right?

[90GTVert]

Yeah, a leaking head gasket or bad threads on the spark plug hole could do it. You should notice the threads of the spark plug hole. Usually you'll see some type of residue outside the cylinder on the deck or head if a head gasket is leaking significantly.